It is well known that thermal and photochemical yellowing are major concerns in magenta image stability of color prints. Over the years improvement in magenta image stability has been achieved by introducing more efficient image stabilizers. However, there still exists a need to further improve the resistance to yellowing in color paper.
It has been known for some time that compounds having the generic structure S are able to undergo reaction with residual magenta coupler and thereby effectively prevent both thermal and photochemical yellowing since the products of the reaction are not yellow and are not prone to yellowing. However, a major problem in the utilization of these compounds is the loss of coupler during storage of the photographic element prior to exposure and processing resulting in a reduction in color density in the print. See for example, U.S. Pat. No. 4,540,657 to Krishnamurthy and Japanese Patent Publication No. 62-131259 to Fuji Photo Film Co., Ltd. The generic structure of Compound S is represented below: ##STR3## wherein: A is a polyvalent atom, an acidic oxide group, a carboxylic group, a heterocyclic moiety, a carbocyclic group, or an alkane or substituted alkane group; each L is at least one divalent linking group; R.sub.1 and R.sub.2 are H, alkyl, cycloalkyl, aryl, heterocyclic, ester; n is a positive integer with a range of 1-6; m is a positive integer of at least one; with the proviso that at least one of A, L, R.sub.1 or R.sub.2 contains at least one ester or amide group derived from an acidic oxide of carbon, phosphorous, sulfur, boron or silicon.
In copending, commonly assigned application U.S. Ser. No. 08/000,431, filed Jan. 4, 1993, we showed that the compound S-1 (having the structural formula set forth below) could be incorporated in a silver halide color photographic element containing a ballasted magenta coupler such that there is negligible loss of coupler prior to processing. This was achieved by coating the epoxy compound in separate layers that were adjacent to the imaging layer containing the magenta coupler and the green sensitized emulsion. Furthermore, it has also been demonstrated that mixing of S-1 with residual magenta coupler after processing may be achieved by using a pH dependent solubilizing agent, e.g., a fatty acid, such as myrisitic acid, in the coating and processing the coating using developer which preferably contains benzyl alcohol. However, the use of benzyl alcohol in the developer raises environmental concerns. ##STR4##
In an effort to eliminate the use of compounds such as benzyl alcohol to achieve post process mixing of the epoxy scavenger with the residual magenta coupler, copending, commonly assigned U.S. Ser. No. 08/255,512, filed Jun. 8, 1994, discloses novel terminal epoxy compounds containing a pH-dependent solubilizing moiety in the molecule. Dispersions of these compounds are coated in layers adjacent to the magenta imaging layer, and the compounds diffuse into the magenta layer upon processing where they react with residual magenta coupler yielding products that are not yellow or prone to yellowing. The approach is effective in reducing thermal and photochemical yellowing in processed prints without the use of benzyl alcohol.
Hydrophobic photographic additives are conveniently incorporated in a hydrophilic colloid layer of a photographic element by dissolving the hydrophobic additive in an organic solvent and then dispersing the organic solution in an aqueous solution to form a homogenized dispersion. The epoxy compounds exemplified in U.S. Ser. No. 08/255,512, however, are generally characterized by a relatively high melting point (greater than 50.degree. C.) and low hydrophobicity (the logarithm of the octanol/water partition coefficient (logP) for these compounds is less than 8.0). The low hydrophobicity is essential to achieve efficient post-process migration of these compounds from the adjacent non-imaging layers into the magenta imaging layer. The combination of low hydrophobicity and high melting point makes it difficult to prepare stable dispersions of these compounds by homogenization in an aqueous solution even if relatively large amounts of water-immiscible organic solvent is used to dissolve the compounds. The exemplified compounds are accordingly dispersed in the examples of U.S. Ser. No. 08/255,512 by a solid particle dispersion process. In this method size reduction of the epoxy compound is achieved by attrition with media such as zirconia beads instead of by shearing action on a liquid solution. The solid particle dispersion process is relatively inefficient compared to homogenization and often requires several days of grinding in order to achieve the desired level of size reduction.
Problem to be Solved by the Invention
It would be most desirable to have ionizable terminal epoxy compounds of low hydrophobicity that could be incorporated in the photographic element as homogenized dispersions. It is towards this end that the present invention is directed.